An oxide semiconductor such as InGaZnO has been mainly applied to a device for a display. An application thereof to a thin film field effect transistor formed on a wiring layer of a large scale integrated circuit (LSI) is also expected. The thin film field effect transistor formed for an LSI is particularly required to reduce the size thereof.
Therefore, as in a conventional silicon transistor, it is preferable to form a top gate structure in which a source/drain region can be formed by self alignment with respect to a gate electrode. However, unlike the conventional silicon transistor, it is difficult to form a diffusion layer by injecting impurities or to form a metal layer such as a silicide in the oxide semiconductor transistor. Therefore, it is difficult to achieve low-resistance source/drain region with the top gate structure. Consequently, in the top gate structure, a large parasitic resistance of source/drain region become a problem.